Communication devices such as User Equipments (UE) are also known as e.g. Mobile Stations (MS), mobile terminals, and wireless terminals. UEs are enabled to communicate wirelessly in a wireless communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two UEs, between a mobile station and a regular telephone and/or between a mobile station and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications network.
Examples of wireless communication systems are Long Term Evolution (LTE), Universal Mobile Telecommunications System (UMTS) and Global System for Mobile communications (GSM).
UEs may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another mobile station or a server.
The wireless communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a Base Station (BS), e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the UEs within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
Mobile data traffic is growing exponentially due to an enormous success of smart phones, tablets and other data traffic appliances. A traditional way to increase the data rate in wireless communication networks has been to increase the transmission bandwidth. However, the available spectrum has become scarce due to an increase in wireless access systems. Hence a challenge for future wireless access systems is to find alternative solutions to meet high demands on the data rate.
One way of handling increased wireless data traffic in the wireless communications network is to deploy more BSs and densify the wireless communications network. However, this will increase interference and deployment cost.
Another, seemingly simpler option in terms of deployment cost, for increasing the data rate is to introduce more antennas at the base station. By using more antennas at a transmitter, it is possible to direct a transmitted signal to intended user equipment. This is known as beamforming in the literature. Additionally, with many base station antennas it is also possible to simultaneously transmit payload data to multiple user equipment on the same frequency and/or time resources. This is often referred to as multi-user scheduling.
When performing beamforming, the transmitter needs to have some kind of channel knowledge such that it knows how to shift the amplitude and phase of the different antenna elements. In LTE, this channel knowledge may be obtained through channel reciprocity or through Channel State Information (CSI) reports. In for example Transmission Mode (TM) 10, the UE is set up to measure on certain reference signals, CSI Reference Signals (CSI-RS). The UE then reports the CSI in the uplink in the form of a Channel Quality Indicator (CQI), a Rank Indicator (RI) and a Precoding-Matrix Indicator (PMI). The PMI indicates which precoder to use at the transmitter, that is, which weights to put on the antenna elements such that the energy is transmitted towards the UE.
A different way to obtain the channel knowledge is to transmit different beams and let the UE measure on those and report back one or more preferred beams. In the case of reporting the best beam, the UE would only need to indicate which beam to choose from instead of a precoder index indicating phase shifts. This means that the UE only has to measure the signal strength of the beam rather than trying out different phase shifts and estimate the corresponding quality. A cell, or in other words the base station, may send out a large number of beams to measure on in all possible directions. However, this costs significant energy and overhead in the cell.
Moreover, for high mobility UEs, and/or narrow data beams, the antenna gain may quickly drop such that every time the base station receives a feedback, it is to some extent no longer valid, thus making a beamforming choice less effective. In other words, as a reported CSI quickly may become outdated with a high mobility user equipment, the user equipment risks receiving the data sent through a beam which is not best suited for the instantaneous position but rather for a previous position.